Porous sound absorbers

explained   

Most (99% actually) materials used in construction to absorb sound are porous.

And do you know:

 

  • the 3 differents types of porous absorbers?
  • the importance of their porosity and air flow resistivity?
  • the influence of their thickness
  • the influence of their density?
  • the influence of the mounting conditions?
  • the impact of the materials used to cover them?

 

In this post, we answer all the above questions.   

Enjoy the read!

(See at the end the references used to confirm the  concepts)

 

You want to be notified when new posts are published? Why don’t you subscribe to Atelier Crescendo’s newsletter by clicking here?

Are you looking to find examples of acoustic solutions and products available on the market?

Visit the Acoustic Design Catalogue by clicking on the button below.

 

access the catalogue here

What are porous sound absorbers


Primarily, Porous absorbers are used to diminish sound reverberation in rooms or to mitigate noise transmission when installed in cavities.

The sound absorption is achieved when sound passes through networks of interconnected pores, inducing viscous and thermal effects that dissipate acoustic energy as heat.

There are three types of porous absorbers.

 

 

Fibrous

absorbers

The sound absorption properties of fibrous absorbers stem from the arrangement of overlapping fibers, creating interconnected air pockets (the pores).

 

Examples of fibrous absorbers include:

mineral wool

wood fibre

hemp

textile fibres

felt

wool

 

Cellular

absorbers

The sound absorption properties of cellular absorbers arise from pores either isolated from their neighbours (known as “closed” pores or cells) or interconnected with their neighbours (known as “open” pores or cells).

Examples of cellular absorbers include:

PU foam

PET foam

 

GRANULar

absorbers

The sound absorption properties of granular absorbers originate from the air pockets (the pores) formed between tiny grains densely packed together.

Examples of granular absorbers include:

concrete

gravel

soils

 

 

Note

The shape and dimensions of the pores and fibres also significantly influence the sound absorption

 

 

 

Porosity and air flow resistivity of porous sound absorbers ? 


The sound absorption of a material is closely tied to its porosity and flow resistivity, representing the  resistance of the air as it traverses the material.

Hence, higher porosity correlates with greater sound absorption.

Porosity is defined as the ratio of pore volume to the matrice/medium volume.

 

Porosity of some construction materials

Material

Typical porosities (%)

Felt 

0,83 – 0,97

Hemp

0,99

Mineral wool

0,92 – 0,99

Open cell acoustic foams 

(e.g. PU)

0,93 – 0,995

Rubber crumb

0,44 – 0,54

Sand

0,39 – 0,44

Marble

≈ 0,005

 

 

Influence of the thickness on the sound absorption? 


When located on solid/hard surfaces, the sound absorption tends to increase with the thickness of the porous material, particularly at low and mid frequencies.

See below the sound absorption of fibreglass (53 kg/m3 density) samples of different thicknesses, fixed to a hard surface.

 

Sound absorption of fibreglass (53 kg/m3 density) samples of 25 mm, 50 mm, 75 mm and 100 mm thicknesses (fixed to a hard surface)

 

 

Why is that

With increasing thickness, more material exists away from the surface, where the particules’ velocity (i.e. energy) is higher than at the wall.

This leads to enhanced absorption, particularly for lower frequencies (or larger wavelengths).

If you need a refresher about frequencies and wavelengths, read this post

 

 

 

Influence of the density on the sound absorption? 


Similarly to the thickness, the sound absorption at low and mid frequencies typically increases with the density of a fibrous absorber.

Below are the sound absorption coefficients of mineral wool blanket samples with varying densities.

 

Sound absorption of mineral wool of 16 kg/m3 , 24 kg/m3, 48 kg/m3 and 60 kg/m3 densities (50 mm fixed to a hard surface)

Influence of the mounting conditions on the sound absorption? 


The sound absorption of a material greatly depends on how it is mounted.

Remember the post about Sound absorption basics? We introduced ISO EN 354 (2003 – Acoustics Measurement of sound absorption in a reverberation room mention) setting measurement standards for various mounting conditions.

For instance, consider acoustic ceiling tiles, which can be either directly fixed to a soffit or suspended.

While the sound absorption is comparable at mid and high frequencies, it is notably higher at low frequencies when the ceiling tiles are suspended (see the graph below).

 

Sound absorption of acoustic tiles directly applied to a hard surface or suspended 

Also, in Suspended acoutic rafts explained, we showed that the following aspect influence the sound absorption characteristics a raft systems:

 

  • the sound absorption of the material installed in a room 
  • the size of the rafts
  • the spacing between the rafts
  • the distance between the rafts and the hard surface above.

 

Influence of the cover on the sound absorption? 


In many cases, sound absorbers are covered or protected by materials such as:

 

  • fabric
  • felt
  • plastic
  • paint
  • perforated sheet material
  • slats

 

Some of these materials possess specific air flow resistivity and may prevent sound from passing through. Thereby reducing the sound absorption especially at the higher frequencies. Less so at low frequencies.

This is why some materials, like paint, should be avoided as they block the pores of the materials (unless it is a non-bridging paint). See below different performances for different paint applications. 

 

Influence of paint treatment on the absorption of a sound absorber

 

The same applies to glue behind cloths.

Cloth, perforated sheet materials and slats are typically preferred for protecting the sound absorbers against impact and/or small particles.

References

 

See below the references used to confirm the above concepts: 

 

  • Noise Control in Building Services – Sound Research Laboratories Ltd – Pergamon Press
  • Sound Materials, A Compendium of Sound Absorbing Materials for Architecture and Design – Tyler Adams – Frame Publishers
  • Acoustic Absorbers and Diffusers – Theory, Design and Application – Third Edition –  Trevor Cox, Peter D’Antonio
  • Engineering Noise Control – Sixth Edition – CRC Press – David A. Bies, Colin H. Hansen, Carl Q. Howard, Kristy L. Hansen

 

Class A Class B Class C Class D Class E - practical sound absorption coefficient - sound absorption class calculation - sound absorption - acoustics - acoustic consultant

Sound absorption classes

explained   

Sound absorption classes are often used to specify sound absorption materials to be installed in buildings.

But have you ever wondered what they are exactly, what they represent and how they are established/calculated?

If you want to know, this short article has been created for you, with some graphics to make you understand the calculation process

Enjoy the read!

 

You want to be notified when new posts are published? Why don’t you subscribe to Atelier Crescendo’s newsletter by clicking here?

Are you looking to find examples of acoustic solutions and products available on the market?

Visit the Acoustic Design Catalogue by clicking on the button below.

 

access the catalogue here

What are sound absorption classes


The classification is a method, among others, to categorise the sound absorption performance of absorbers, particularly for construction materials.

 

 

Note

You can find the details of this classification in ISO 11654 (2023) – Sound absorbers for use in buildings – Rating of sound absorption.

 

 

It is simpler and easier for non-acousticians to grasp since it doesn’t involve numbers.

There are five classes ranging from Class E (lowest performance) to Class A (highest performance).

How are sound absorption classes established


Establishing the sound absorption class of a material involves two main steps:

 

  • Converting the frequency-dependent sound absorption performance to a single-number value.
  • Assigning the single-number performance to the corresponding class.

 

From frequency-dependent performance to single number value

 

Sound absorption data is typically measured in third-octave bands.

(if you need to understand about frequencies read this article, and frequency bands read this article)

The initial conversion involves transitioning from the sound absorption coefficient measured in third-octave bands to the Practical Sound Absorption Coefficient (αp) in octave bands (as introduced in this article).

From there, we calculate the Weighted Sound Absorption Coefficient (αw), which is a single number determined as detailed below.

We begin with two curves:

 

  • the Practical Sound Absorption Coefficient (αp) values calculated (in yellow below).
  • A curve known as the reference curve, with specific values for each octave band between 250 Hz and 4000 Hz (in black below).

 

The second step involves calculating the “unfavourable deviations” for each octave band.

 

 

Note 1:

An unfavorable deviation occurs when, for a given octave, the measured value is

less

than the value of the reference curve.

 

And vice versa, a ‘favourable’ deviation occurs when, for a given octave, the measured value is

more

than the value of the reference curve.

 

Note 2:

In the calculations, we only consider

unfavourable deviations.

 

 

Then, we incrementally adjust the reference curve in 0.05 steps toward the measured value until the sum of unfavorable deviations is ≤ 0.10.

 

Once this criterion (≤ 0.10) is met, the performance (αw) for the material is determined by the value of the shifted reference curve at 500 Hz.

Another important point to note is if a measured value exceeds the reference value by more than 0.25 (even after the shifting process), a shape indicator is added to the weighted sound absorption performance as follows:

 

  • L, like αw (L), if excess occurs at 250 Hz
  • M, like αw (M), if excess occurs at 500 and 1000 Hz
  • H, like αw (H), if excess occurs at 2000 and 4000 Hz

 

From single number performance to sound absorption class

 

Once the single number is calculated, we associate it with a class using the table below.

 

The figure below presents the reference curves corresponding to each class (you have probably seen this type of graph before). 

 

Sound absorption basics  

Often, we, acousticians, are asked about the fundamentals of sound absorption

We are also asked to explain sound absorption data received by suppliers & manufacturers of sound absorbing materials. 

If you you need clarification on sound absorption, we wrote this article for you. 

It explains 

  • what a sound absorber is
  • how we rate the sound absorption capacity of materials. 
  • how we measure the sound absorption coefficient of materials.

Enjoy the read!

(See at the end the references used to confirm the  concepts)

Are you looking to find examples of acoustic solutions and products available on the market?

Visit the Acoustic Design Catalogue by clicking on the button below.

 

access the catalogue here

What is a sound absorber


A succinct definition of a sound absorptive material is:

 

“A sound absorptive material is a material that acts as a ‘noise sponge‘ by converting the sound energy into heat within the material.”

(from  Environmental Impact Assessment, Theory and Practice – Anji Reddy Mareddy)

 

When sound strikes a material, some of its energy converts to heat, reducing the reflected sound energy.

 

Often, we, acousticians, are asked about the fundamentals of sound absorption. We are also asked to explain sound absorption data received by suppliers & manufacturers of sound absorbing materials. If you you need clarification on sound absorption, we wrote this article for you. It explains what a sound absorber is how we rate the sound absorption capacity of materials. how we measure the sound absorption coefficient of materials. Enjoy the read! - Reflected sound - with less energy - sound absorber - absorbed sound - incident sound

 

The sound absorption of construction materials typically spans across narrow or wide frequency ranges, not single frequencies.

(if you need to understand what frequencies are read this article, and frequency bands read this article)

Various factors influence a material’s sound absorption in a space:

 

  • the material type (hard, fibre-based, porous, etc)
  • the thickness
  • the finish (painted, cloth-covered, plastered, etc)
  • the placement in the space
  • the mounting method (on/off hard surface, free-hanging, etc)

 

Common sound absorptive solutions in construction use porous, fibrous, or cellular materials.

Other solutions utilize these materials with additional features like cavities, perforations or microperforations to enhance the sound absorption across specific frequency ranges.

Examples of materials will be presented in future posts.

 

How do we rate the sound absorption capacity of materials


For most construction materials, a common index to rate the sound absorption is the sound absorption coefficient , termed α (‘alpha’).

 

 

Note

It is officially called the Practical Sound Absorption Coefficient, termed αp, in the standard ISO 11654 (2023) – Sound absorbers for use in buildings – Rating of sound absorption.

 

 

 

This coefficient is the ratio of: 

 

  • the sound energy absorbed (Ea) by a material; 
  • divided by the sound energy incident (Ei) on the same material.

For most construction materials, a common index to rate the sound absorption is the sound absorption coefficient , termed α ('alpha'). Note: It is officially called the Practical Sound Absorption Coefficient, termed αp, in the standard ISO 11654 (2023) - Sound absorbers for use in buildings - Rating of sound absorption. This coefficient is the ratio of:  the sound energy absorbed (Ea) by a material;  divided by the sound energy incident (Ei) on the same material. incident sound energy - absorbed sound energy - sound absorption coefficient

 

As the absorbed sound energy (Ea) increases, the sound absorption coefficient increases too.

Conversely, a decrease in absorbed sound energy results in a lower absorption coefficient.

Thus, better sound absorption yields a higher absorption coefficient, logically.

 

 

Note

It’s worth also noting that Ea can’t physically be higher than Ei (there can’t be more absorbed energy than there is incident energy).

Hence Ea ≤ Ei always, ensuring that the sound absorption coefficient is always between 0 and 1.

However, coefficients higher than 1 may occur due to testing methods or calculations.

 

 

 

Other methods and indices for evaluating sound absorption of building materials include:

 

  • the Noise Reduction Coefficient (NRC) and Sound Absorption Average (SAA) as per ASTM C423 – Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method
  • the Weighted Sound Absorption Coefficient (αw) as per ISO 11654 (2023).
  • the Sound absorption class (ranging from E to A, with A being the highest), as per ISO 11654 (2023)
  • the Equivalent sound absorption area.

 

The latter two will be elaborated in future posts.

 

 

How do we measure the sound absorption coefficient of materials


There are two main methods to measure the sound absorption coefficient of a material: 

 

  • the Impedance Tube Method.
  • the Reverberation Chamber Method or Room Method.

 

The Impedance Tube Method 

 

This method involves placing a sample of material at the end of a small tube.

Sound is emitted into the tube at different frequencies.

The sound absorption coefficient of the material is then derived by comparing the sound in the tube before and after it strikes the sample.

 

The Impedance Tube Method  This method involves placing a sample of material at the end of a small tube. Sound is emitted into the tube at different frequencies. The sound absorption coefficient of the material is then derived by comparing the sound in the tube before and after it strikes the sample. While more practical and time-efficient, this method is not preferred due to its restrictions on the type of material and mounting conditions.  Note: Under the impedance tube method, there are actually two sub-methods: the standing wave ratio method and; the transfer function method. Both are prescribed by normative regulations outlined in the following standards: ISO 10534-1 (2022) - Determination of sound absorption coefficient and impedance in impedance tubes - Part 1: Method using standing wave ratio. ISO 10534-2 (2023) - Determination of sound absorption coefficient and impedance in impedance tubes - Part 2: Transfer-function method. ASTM E 1050-19 - Standard test method for impedance and absorption of acoustical materials using a tube, two microphones and a digital frequency analysis system. sound source - reflected sound - microphones - tube - rigid sound reflective end - incident sound

 

While more practical and time-efficient, this method is not preferred due to its restrictions on the type of material and mounting conditions.

 

 

Note

Under the impedance tube method, there are actually two sub-methods:

  • the standing wave ratio method and;

  • the transfer function method.

 

Both are prescribed by normative regulations outlined in the following standards:

  • ISO 10534-1 (2022) – Determination of sound absorption coefficient and impedance in impedance tubes – Part 1: Method using standing wave ratio.

  • ISO 10534-2 (2023) – Determination of sound absorption coefficient and impedance in impedance tubes – Part 2: Transfer-function method.

  • ASTM E 1050-19 – Standard test method for impedance and absorption of acoustical materials using a tube, two microphones and a digital frequency analysis system.

 

 

 

The Reverberation Chamber or Room Method 

 

This method involves placing large samples or multiple samples of the material to be tested in a large &  highly reverberant room.

The room typically comprises hard surfaces (walls, ceiling, and floor) that reflect sound, absorbing minimal to no sound, along with large sound reflective surfaces hung from the ceiling and sometimes positioned near the walls.

 

This method involves placing large samples or multiple samples of the material to be tested in a large &  highly reverberant room. The room typically comprises hard surfaces (walls, ceiling, and floor) that reflect sound, absorbing minimal to no sound, along with large sound reflective surfaces hung from the ceiling and sometimes positioned near the walls.

 

This setup aims to scatter sound extensively, creating a diffusive sound field within the room. This enables simulation of sound reaching the samples from all directions.

The sound absorption coefficient is calculated by comparing:

  • Sound reverberation measurements without any materials.
  • Sound reverberation measurements with the material under test.

 

This setup aims to scatter sound extensively, creating a diffusive sound field within the room. This enables simulation of sound reaching the samples from all directions. The sound absorption coefficient is calculated by comparing: Sound reverberation measurements without any materials. Sound reverberation measurements with the material under test.

 

 

Note

This measurement methodology is prescribed in EN ISO 354 (2003) – Acoustics Measurement of sound absorption in a reverberation room mention.

 

 

 

While not as practical or time-effective as the impedance tube method, this approach offers the advantage of testing materials in more realistic mounting conditions. These include (but not only):

 

  • Mounted directly against a hard surface

While not as practical or time-effective as the impedance tube method, this approach offers the advantage of testing materials in more realistic mounting conditions. These include (but not only): Mounted directly against a hard surface. 

 

  • Hung freely.

While not as practical or time-effective as the impedance tube method, this approach offers the advantage of testing materials in more realistic mounting conditions. These include (but not only): Hung freely

 

  • Positioned within a cavity.

While not as practical or time-effective as the impedance tube method, this approach offers the advantage of testing materials in more realistic mounting conditions. These include (but not only): Positioned within a cavity

 

  • Split into small samples

While not as practical or time-effective as the impedance tube method, this approach offers the advantage of testing materials in more realistic mounting conditions. These include (but not only): Split into small samples

 

  • And many others

 

 

 

Note

EN ISO 354  also provides guidance (in Appendix B) on the

appropriate mounting of the test sample whether it is:

  • mounted directly against a surface of the reverberant room (Types A and B).

  • mounted with an airspace behind it (Type E)

  • hung parallel to a surface of the reverberant room (Type G)

  • spray- or trowel-applied to a substrate , usually sound absorptive (Type I)

  • with one edge resting on or touching a room surface (Type J)

.

 

 

 

We will provide examples of sound absorption coefficients for different construction materials in future posts.

 

References

 

See below the references used to confirm the above concepts:  

  • Noise Control in Building Services – Sound Research Laboratories Ltd – Pergamon Press
  • Sound Materials, A Compendium of Sound Absorbing Materials for Architecture and Design – Tyler Adams – Frame Publishers
  • Acoustic Absorbers and Diffusers – Theory, Design and Application – Third Edition –  Trevor Cox, Peter D’Antonio
  • Engineering Noise Control – Sixth Edition – CRC Press – David A. Bies, Colin H. Hansen, Carl Q. Howard, Kristy L. Hansen
  • Mechanical and Physical Testing of Biocomposites, Fibre-Reinforced Composites and Hybrid Composites –  (A volume in Woodhead Publishing Series in Composites Science and Engineering) Mohammad Jawaid, Mohamed Thariq & Naheed Saba – Woodhead Publishing – 2019

 

 

Sound propagation, reflection, absorption and transmission 

Quite often, we receive questions on how sound propagates, especially through materials

If you need to understand the difference between sound reflection, sound absorption and sound transmission, here is a small post to explain the principles. 

Enjoy the read!

(See at the end the references used to confirm these  concepts)

PS: if you need to understand some basic concepts of sound like wavelengths and frequencies, read fundamental concepts of sound.

 

How does sound propagate


Assuming a single sound source, typically called a point source, emitting sound uniformly (i.e. the same way in all directions).

In the near field, it produces sound waves that spread outward from the source in a spherical pattern

 

Note: the near field is a region close to the source

 

In the far field, sound continues to propagate outward from the source. However, it appears to propagate more in a line pattern.

 

Note: the far field is a region far from the source

 

 

 

To aid visualization, we often use rays to represent how sound propagates. They clearly indicate the direction of a specific sound’s propagation.

Throughout the remainder of this post, as well as in many others, we illustrate sound propagation using rays.

 

What are sound reflection, absorption and transmission?

(and also sound insulation and soundproofing) 


Assuming a material that is infinitely long and flat, with no thickness considered.

When sound propagates at a certain angle from the surface, the angle between its direction and the surface’s normal is called the angle of incidence.

 

 

 

As sound interacts with this material, three phenomena occur.

Sound is reflected

 

 

at the same angle as the angle of incidence.

We call it the angle of reflection.

 

Sound is absorbed

 

 

losing acoustic energy transformed into heat within the material.

 

Sound is transmitted

 

 

to the other side of the material.

 

A material’s ability to reduce the sound transmission is its sound insulation capacity

(or also soundproofing capacity).

This capacity varies depending on the material’s physical properties, affecting how sound is reflected, absorbed, and transmitted.

Additionally, sound reflection, absorption, and transmission vary across different frequencies.

(if you need to understand the concept of frequencies, read the fundamental concepts of sound)

 

References

See below the references used to confirm the above concepts:  

  • Noise Control in Building Services – Sound Research Laboratories Ltd – Pergamon Press
  • Sound Materials, A Compendium of Sound Absorbing Materials for Architecture and Design – Tyler Adams – Frame Publishers
  • Acoustic Absorbers and Diffusers – Theory, Design and Application – Third Edition –  Trevor Cox, Peter D’Antonio
  • Engineering Noise Control – Sixth Edition – CRC Press – David A. Bies, Colin H. Hansen, Carl Q. Howard, Kristy L. Hansen

 

Understanding frequency bands

In fundamental concepts of sound, we covered some basic concepts like wavelengths and frequencies.

Acoustic experts often use frequency bands in their studies.

For non-experts, understanding concepts like third-octave and octave frequency bands can be tricky.

If you find this concept challenging, worry not! Atelier Crescendo has crafted this post just for you.

Enjoy the read!

(See at the end the references used to confirm these  concepts)

 

Acoustic analysis 


Studying the acoustic performance of materials or the acoustic characteristics of sources/signals comes in different ways.

Frequently, acoustic experts use frequency bands for measurements or calculations.

Essentially, it is a method to reveal acoustic information across different frequency ranges.

Most commonly, frequency analysis occurs in:

  • octave bands 
  • third-octave bands

In third-octave bands, the precision/resolution is about three times higher than in octave bands.

The formula to obtain octave band values from third octave values depends on the acoustic metric you want to study.

What is an octave ?


An octave is an (key musical) interval, corresponding to doubling or halving frequency.

 

Example: from 125 Hz to 250 Hz is an octave, as is 20,000 Hz to 10,000 Hz.

 

Musicians often use the A note as the reference for tuning. The frequency of this note is 440 Hz.  The A note at the next octave is 880 Hz

 

 

Upper & higher frequency band limits

and band centre frequency  


When talking about bands, we must set their upper and lower frequency limits.

Also, in each band, there is a center, known as the band center frequency. It is the mean of the upper and lower limits.

 

Example: in the 250 Hz center frequency octave band:

  • the lower limit is 178 Hz
  • the upper limit is 355 Hz 

 

 

Setting limits for third-octave bands is a little more complex.

 

Preferred (and standardised) frequency bands


Frequency band analysis has even been standardised

International Standards Organisations have agreed on preferred frequency bands for the analysis of sound or vibrations.

Typically, in acoustic building projects, the frequency range under consideration spans from 63 Hz to 8000 Hz.

In the case of vibrations, exploration can extend to frequencies as low as 2 Hz.

Note: If you want to know about the standards, here they are: 

  • ANSI/ASA S1.6 (2020) – Preferred Frequencies And Filter Band Center Frequencies For Acoustical Measurements
  • ANSI/ASA 51.11 (2004) – Octave-Band And Fractional-Octave-Band Analog And Digital Filters
  • IEC 61260 -1 (2014) – Electroacoustics – Octave-band and fractional-octave-band filters – Part 1: Specifications
  • TS0 266 (1997)  – Acoustics – Preferred frequencies

 

References

See below the references used to confirm the above concepts:  

  • Noise Control in Building Services – Sound Research Laboratories Ltd – Pergamon Press
  • Sound Materials, A Compendium of Sound Absorbing Materials for Architecture and Design – Tyler Adams – Frame Publishers
  • Acoustic Absorbers and Diffusers – Theory, Design and Application – Third Edition –  Trevor Cox, Peter D’Antonio
  • Engineering Noise Control – Sixth Edition – CRC Press – David A. Bies, Colin H. Hansen, Carl Q. Howard, Kristy L. Hansen

 

Deep into wavelengths, frequencies and spectrums of instruments

 

This post is not very long.

It is a complement to the fundamental concepts of sound in which we explained about sound, sound waves, amplitude, wavelengths and frequencies.

Here, we firstly give you some ideas on the dimensions of wavelengths for different frequencies in the air. They are useful to know to better understand sound phenoma like: sound absorption, sound diffusion, sound diffraction and also sound transmission. 

Then we have included a graph with examples of spectrum of some instruments and sources. This way, you can visualise the frequencies they can emit. 

Wavelength dimensions


Below we illustrate wavelength dimensions for various frequencies within the audible range for humans.

These examples offer insights into the sizes of wavelengths associated with the sounds/frequencies we can hear.

 

 Note: We have approximated the wavelength values by calculating them under the assumption of air as the propagation medium, at a temperature of 20 degrees Celsius, resulting in a speed of sound at 343 meters per second.”

 

Examples of wavelength dimensions corresponding to different frequencies

Frequency

Corresponding Wavelength

20 Hz 

17,15 meters

50 Hz

6,86 meters

100 Hz

3,43 meters

500 Hz

69 cm

1000 Hz

34 cm

5000 Hz

6,9 cm

10000 Hz

3,4 cm

20000 Hz

1,7 cm

Examples of spectrums


Remember from the fundamental concepts of sound, “the natural sounds blend frequencies with varying amplitudes.” 

We call spectrum (in physical terms) the range of frequencies a source can emit.

Below we have included a graph with examples of spectrums of some instruments and sources. 

 

 

 

Fundamental concepts of sound

(Sound waves, amplitude, wavelength, frequency, etc)

Are you new to acoustics or need a refresher on sound basics?

This article was created to dive into the fundamental concepts of sound including:

 

  • Sound and sound waves
  • Sound wavelength
  • Sound amplitude
  • Sound frequency

 

Enjoy the read!

(See at the end the references used to confirm these  concepts)

 

Understanding sound and sound waves


Sound originates from vibrating objects in various mediums like solids, liquids, and gases.

These vibrations cause air particles to move back and forth, creating compression and rarefaction.

 

Compression occurs when particles bunch together, generating a higher pressure.

Conversely, rarefaction leads to lower pressure as particles spread apart.

These alternating pressure fluctuations give rise to the sounds we perceive.

These pressure variations form sound waves, classified as longitudinal waves because the air particles move in the same direction as the vibrations.

In air, sound waves travel at around 343 meters per second (at 20°C).

Understanding wavelength in sound waves


Wavelength in sound waves refers to the distance between two consecutive compressions or rarefactions of particules.

Typically, wavelengths are expressed in meters.

 

 

Understanding amplitude


Sound amplitude reflects the change in pressure from vibrations. Put simply, it relates the number of air particles involved in the vibration process.

A sound with a greater amplitude will be perceived as louder (or with a higher volume).

 

To measure amplitude, you use logarithmic decibels (dB).

For instance, a whisper might register around 30 dB, while a rock concert can reach a staggering 120 dB or higher.

Understanding frequencies 


Frequencies, measured in Hertz (Hz), directly relate to sound wave speed divided by the wavelength.

  • shorter wavelengths correspond to higher frequencies…

 

  • while larger wavelengths align with lower frequencies

Higher frequencies result in higher pitches, while lower frequencies create lower pitches. Listen below to some examples perfect pitches at different frequencies.

 

100 Hz pitch

500 Hz pitch

1000 Hz pitch

10000 Hz pitch

The audible range for most humans falls between 20 Hz to 20,000 Hz.

Natural sounds blend frequencies with varying amplitudes, forming a spectrum.

The unique timbre of sources, especially musical instruments, links closely to their individual spectra.

 

References

See below the references used to confirm the above concepts:  

  • Noise Control in Building Services  – Sound Research Laboratories (SRL) Ltd – Pergamon Press
  • Auditorium Acoustics and Architectural Design (Second Edition) – Michael Barron – Spon Press
  • Sound Materials, A Compendium of Sound Absorbing Materials for Architecture and Design – Tyler Adams – Frame Publishers

 

Noise and vibration challenges in sport, fitness and gym facilities

Part 2: Control of airborne noise

 

 

 

 

In sport, fitness and gym facilities, many activities and sources generate airborne noise including: 

 

  • Power amplified sound systems 
  • Exercise machines
  • Weights dropped (controlled or uncontrolled) on structures
  • People chatting, cheering, shouting, etc. 
  • Other noises specific to some sports (i.e. hockey, tennis, football, etc.)

 

This part explains the different paths of airborne noise transmission from the gym spaces to the adjacent/nearby noise sensitive receptors.

For each transmission path, it explains what happens and gives ideas of noise control solutions. 

 

Note 1: what is airborne noise exactly? go to this section  in part 1.

 Note 2 : read Who could the facility disturb iin part 1 to know what the noise sensitive receptors could be. 

 

 

 

Airborne noise transmission paths in sport, fitness and gym facilities

The main transmission paths of airborne noise are:

  • through the building façace.
  • through the internal building fabric and;
  • via the ventilation systems.  

It is also useful to control the sound reverberation in noisy spaces.

Follow the links to read what happens and know ideas of noise control solutions for each transmission path. 

 

Noise transfer through the building façade 

of sport, fitness and gym facilities

What happens?


 

 

In sport, fitness and gym facilities, activity and music noise:

 

  • transfers through the façade, 
  • and propagates through the air to the nearest sensitive receptor.

 

Local Authorities often ask to consider this aspect to make sure the new facility doesn’t cause an adverse impact on the neighbours

To do this, you need to undertake a noise impact assessment

 

Note: Most of the time, music and activity noise impact assessments aim to not exceed (to a degree) the existing noise environmental noise levels at the receptors. There are called subjective assessments’

But sometimes, undertaking a subjective assessment is not the most ‘reasonable’ approach

For very quiet sites, exceeding slightly the existing noise environmental noise levels may not cause an adverse impact.

In this case, undertaking an assessment based on not exceeding (again, to a degree) an objective requirement may be more appropriate. This requirement, usually from a guideline or a standard, has to be discussed and agreed with the Local Authority

You call such as assessment an ‘objective assessment’.

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What are the noise control solutions?


There are two main solutions to control music and activity noises in sport, fitness and gym facilities.

Solution 1:

Improve the façade elements and/or the roof

acoustics - acoustic design - airborne transmission - vibration - gym - gyms - fitness - sport facilities - sports facilities - noise control of gyms - activity noise break out - noise breaking out - amplified music noise

You generally do it by creating cavities with dense & rigid board elements such as:

 

  • plasterboard lining on a frame with insulation in the cavity to improve the performance of the external walls.

You may need to decouple it from the rest of the building structure by introducing resilient fixings or, when possible, simply make the frame independant.

Examples of resilient fixings are resilient bars or resilient clips

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  • plasterboard ceiling on a frame with insulation in the cavity to improve the performance of the roof.

You may need to decouple it from the rest of the building structure by introducing resilient fixings.

As above, examples of resilient fixings are resilient  bars, clips or hangers.

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  • if the façade contains glazed elements, you may need to think about high performance double or triple glazing

Sometimes, secondary glazing is necessary when you need to increase the sound insulation performance at low frequencies.  

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Solution 2:

Install a noise limiter

 

acoustics - acoustic design - airborne transmission - vibration - gym - gyms - fitness - sport facilities - sports facilities - noise control of gyms - activity noise break out - noise breaking out - amplified music noise - noise limiter

 

A noise limiter is usually a secondary solution. 

It is a device that limits (!!) the music levels emitted within a space to avoid too much noise spilling out of a facility. 

This way the music levels stay below certain thresholds at the nearest noise sensitive receptors. 

 

Note: the word thresholds here is with an “s” because you set the noise limiter in different frequency bands (or ranges).  

 

 

 Noise transfer through the internal building fabric 

of sport, fitness and gym facilities

What happens?


acoustics - acoustic design - airborne transmission - vibration - gym - gyms - fitness - sport facilities - sports facilities - noise control of gyms - activity noise break out - noise breaking out - amplified music noise - noise transfer through the internal building fabric of sport , gym and fitness facilities

In a building, the airborne noise transfers through the wall/floor constructions into adjacent spaces.

What are the noise control solutions?


Depending of the site and the context, you may have to implement some or all the following noise control solutions.

Solution 1:

Improve the floors

acoustics - acoustic design - airborne transmission - vibration - gym - gyms - fitness - sport facilities - sports facilities - noise control of gyms - activity noise break out - noise breaking out - amplified music noise - noise transfer through the internal building fabric of sport , gym and fitness facilities - improve the sound insulation of the floors

 

You can improve the airborne sound insulation performance of a floor construction with:

 

  • a dense suspended ceiling that you decouple from the main structure with resilient fixings such as resilient bars, clips or hangers.

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Note: you may also need to consider a floating floor system in the gym. 

However, this is more to control the impacts of weights dropped and vibrations of some activities & equipment (see future parts on vibration isolation).

Solution 2:

Improve the walls

acoustics - acoustic design - airborne transmission - vibration - gym - gyms - fitness - sport facilities - sports facilities - noise control of gyms - activity noise break out - noise breaking out - amplified music noise - noise transfer through the internal building fabric of sport , gym and fitness facilities - improve the sound insulation of the walls

 

You can improve the airborne sound insulation performance of external wall constructions in vary many different ways.  

However,  these two systems are recurrent when receptors are fairly close (< 20m) to the facilities :  

 

  • Drylining or ‘sandwichsystems. Most of the time, they include plasterboard on the indoor side. 

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  • Dense linings to masonry walls.

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For both methods, it might be useful to think about introducing:

 

  • a larger cavity to better control low frequency transmission. acoustics - acoustic design - airborne transmission - vibration - gym - gyms - fitness - sport facilities - sports facilities - noise control of gyms - activity noise break out - noise breaking out - amplified music noise - noise transfer through the internal building fabric of sport , gym and fitness facilities - plasterboard - drylining - dense plasterboard lining - increase the size of the cavity acoustics - acoustic design - airborne transmission - vibration - gym - gyms - fitness - sport facilities - sports facilities - noise control of gyms - activity noise break out - noise breaking out - amplified music noise - noise transfer through the internal building fabric of sport , gym and fitness facilities - plasterboard - drylining - dense plasterboard lining - increase the size of the cavity
  • some decoupling elements to increase their performance such as
    • acoustic studs
    • resilient clips
    • resilient bars
    • or even make the studs independant from the outer structure.

Note: If the facility is to move in an existing building, sound insulation testing are necessary to rate the performance of the separations elements and work out the best solution for improvement.

Note: sometimes, a full box-in-box construction will be necessary

 

Noise transfer via the ventilation systems 

of sport, fitness and gym facilities

What happens?


The music and activity noises from sport, fitness and gym facilities:

 

  • ‘enter’ the ventilation systems
  • propagate through the ducts, and;
  • break outside or in another space within the same building (although this last one is unlikely because most facilities have their dedicated ventilation system).

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What are the noise control solutions?


To control the noise transfer via a ventilation system, you can think about the following solutions. 

Solution 1:

Install attenuators

acoustics - acoustic design - airborne transmission - vibration - gym - gyms - fitness - sport facilities - sports facilities - noise control of gyms - activity noise break out - noise breaking out - amplified music noise - noise transfer via the ventilation systems of sport , gym and fitness facilities - building services - attenuator

 

This is probably the most common and easiest solution.

Their performance mainly depends on their length and their free area.

However, be aware that attenuators shouldn’t be installed anywhere in the ductwork for optimal acoustic performance.

The best locations to install the attenuators are:

 

  • centered in the wall separating the noisy space and the rest of the building;

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  • or at the wall separating the noisy space and the rest of the building.  

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Solution 2:

Strategically locate the inlets and outlets

 

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Ideally, you should locate the inlets and oulets of a ventilation system as far away from the sensitive receptors as possible. (although some sites/buildings don’t offer a great amount of flexibility).

This way, you can reduce the performance of the attenuators (and save some cost)… or completely omit them

 

 

Solution 3:

Select larger and square ducts (for low frequency attenuation)

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If you need to attenuate low frequency sounds, you could think about selecting large and square ducts. 

 

Note:  This is only worth if:

  • you have long runs of ductwork.

  • you have enough space for large ducts.

 

 

Solution 4

Select lined ducts

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Lined ducts attenuate more noise than unlined ducts.

Therefore, you can use them to attenuate some amount of music and activity noise. 

 

Note: it is only worth if:

  • you have long runs of ductwork.
  • the pressure drop they create doesn’t require a higher air flow (which would be counter productive). This needs to be checked with the mechanical engineer on board. 

 

 

Control of sound reverberated (i.e. reflected)

in sport, fitness and gym spaces

What happens?


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Within most sport, fitness and gym spaces, medium to high levels of sound are generated.

With a majority of hard surfaces (i.e. sound reflecting), the spaces can be very reverberant and amplify these sounds.

This also makes it harder to control the noise transfer from the gym to other receptors (within or outside of the same building).

What is the noise control solution?


To reduce that reverberation effect, you need to include sound absorptive finishes on the walls and the ceiling.

Where possible, it is useful to install a carpet to contribute to the sound absorption

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Noise and vibration challenges in sport, fitness and gym facilities Part 1: Site Analysis - acoustic consultant - noise control - gym acoustics - vibration control - gym vibration

Noise and vibration challenges in sport, fitness and gym facilities

Part 1: Site Analysis


In sport, fitness and gym facilities, you find a lot of activities and equipment that create noise and vibrations.

If not controlled correctly, they can disburb neighbouring sites, neighbouring premises, and even the facility itself.

Atelier Crescendo have created this series of posts to make you understand:

 

  • which activities are disturbing.
  • how they are disturbing.
  • who / what they can be disturing to.
  • what the solutions are to reduce the noise and vibration disturbance. 

 

This post suggests the first step of such a process: a complete and thorough Site Analysis.

Site analysis of sport, fitness and gym facilities

 

A complete and thorough site analysis of a facility serves the design of efficient and cost effective acoustic solutions.

Therefore, this first post lists a few questions to ask yourself for a site analysis and explains why they are important

 

 

(click on each link to access the relevant section)

 

 

What are the activities proposed?

 

Various activities and equipment in sport, fitness and gym facilities produce noise and/or vibrations.

They do it in very different ways and at different intensities, leading to require very specific control solutions.

This section gives you a few examples of such activities and equipment.


studio - amplified music - Loud amplified music - Noise and vibration challenges in sport, fitness and gym facilities Part 1: Site Analysis - acoustic consultant - noise control - gym acoustics - vibration control - gym vibration

Loud amplified sound

Most fitness and gym facilities play background music which is generally not a problem.

However, loud music (or sounds) may be played in some areas such as:

  • activity and dance studios
  • sports halls during events
  • external sport areas during events
  • others

Therefore, knowing which areas will produce loud amplified sound is important to take into account in the site analysis.

treadmill - elliptical - gym bike - Vibrating machines - Noise and vibration challenges in sport, fitness and gym facilities Part 1: Site Analysis - acoustic consultant - noise control - gym acoustics - vibration control - gym vibration

Vibrating machines

Some equipment that including rotating system (of some sort) not only produce noise but also vibrations. This includes:

  • treadmills
  • bikes
  • ellipticals
  • others

The above equipment produces (relatively) mild but ‘constant’ vibrations that are transmitted to the building structure.

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Resistance machines

Most resistance machines involve lifting a stack of weights indirectly by performing a specific movement. They include:

  • chest press
  • leg press
  • lat pulldown
  • cable biceps/triceps bar
  • seated row
  • shoulder press
  • others

Each time the stack drops, it creates an impact. This impact produces vibrations that are then transmitted to the building structure.

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Functional training and sport areas

Sport, fitness and gym facilities sometimes include functional training and/or sport areas

All these activities involve medium (up to 20-30 kg) or body weights dropped from heights that usually don’t exceed knee height (i.e. about 0.5 m).

They create repeated and (sometimes) synchronised impacts that produce vibrations of ‘medium intensity’.   

They include: 

  • running tracks
  • slam ball/wall ball work
  • sports pitches
  • sports courts (tennis)
  • sled training
  • TRX training
  • kettlebell training
  • others

Free weight areas - barbell - Noise and vibration challenges in sport, fitness and gym facilities Part 1: Site Analysis - acoustic consultant - noise control - gym acoustics - vibration control - gym vibration

Free weight areas

Free weight areas are probably the most challenging areas to isolate in sport, fitness and gym facilities. 

Most of the time, they involve:

  • using medium (up to 20-30 kg) to very heavy (more than 50 kg) weights. 
  • lifting them from knee to above head height.
  • Dropping them on the floor in, most of the time, an uncontrolled manner.

This process creates a very strong impact, with a lot of energy transmitted, that can be very difficult to mitigate

 

Who could the facility disturb? 

(i.e. what are the noise sensitive receptors)

What is a noise sensitive receptor

 

It is a property or a space located within the same building as the gym/sport facility or close to the site.

Examples of such receptors could be:

 

  • Residential properties
  • Commercial properties (i.e. offices, retail, etc)
  • Education facilities
  • Facilities with equipment sensitive to vibrations (such as laboratories, precision engineering facilities, etc)

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One of the reasons why a site analysis is important is to help set relevant target for the acoustic treatment(s) should achieve.  

This target, or also criterion, highly depends on the who/what the facility is likely to disturb

Some noise sensitive receptors are more sensitive than others (see beside what a noise sensitive receptor is). For example:

 

  • dwellings are generally more sensitive to gym noise  than commercial properties (especially at night).
  • Offices are more sensitive than retail premises
  • Some laboratories may have equipment sensitive to levels of vibrations that are not perceptible by the human body.

 

For residential receptors, you usually discuss and agree with the Local Authority. For other receptors, you could agree a criteria with the landlord or the neighbouring facilities.

Therefore, understanding who your sport, fitness and gym facility might disturb will help define the level of noise and/vibration mitigations

 

Where are the noise sensitive receptors? 

Will they be disturbed by airborne noise, structure-borne noise or both? 

 

Knowing where the noise sensitive receptors are allows you to know if the cause of the disturbance is:

Just airborne noise

Note: What is airborne noise? it is noise that only propagates through the air or the atmosphere. 

 

It is the case when the facilities and the receptors are structurally disconnected (i.e. not part of the same building).

 

airborne noise - amplified music - loud music - loud activities - Noise and vibration challenges in sport, fitness and gym facilities Part 1: Site Analysis - acoustic consultant - noise control - gym acoustics - vibration control - gym vibration
Airborne noise of sport, fitness and gym facilities

 

Just  ‘structure-borne’ noise

Note: What is ‘structure-borne’ noise? it is noise generated by vibrations that propagate through a structure (mostly building structures here) and are re-radiated into noise.

 

It is the case when the facilities and the receptors are structurally connected (i.e. they are part of the same building) but are fairly remote from one another (i.e. not adjacent) 

 

Structure-borne noise - structure borne noise - Noise and vibration challenges in sport, fitness and gym facilities Part 1: Site Analysis - acoustic consultant - noise control - gym acoustics - vibration control - gym vibration
Structure-borne noise of sport, fitness and gym facilities

 

Or both airborne noise and ‘structure-borne’ noise (and maybe pure vibration as well):

It is the case when the facility and the receptors are structurally connected and are close to one another (or even adjacent!). 

 

airborne noise - Structure-borne noise - structure borne noise - Noise and vibration challenges in sport, fitness and gym facilities Part 1: Site Analysis - acoustic consultant - noise control - gym acoustics - vibration control - gym vibration
Structure-borne and airborne noise of sport, fitness and gym facilities

 

 

The relevant acoustic treatment to control the noise and/or vibrations from the facility varies for each situation above.

 

 

When will the facility operate? 

Knowing when (i.e. which periods of the day) a sport, fitness and gym facility will operate

is very important to set relevant noise and/vibration control criteria

Choosing to operate a facility, with loud music and/or heavy impacts on the floor, during late evening and night time periods is likely to increase the cost of acoustic treatment needed.

Especially, when the facility is near or adjacent to dwellings

 

Note: Late evening is approximately after 19:00 hours. Night time is approximately between 23:00 and 07:00 hours.

 

The reason of this is because you need to set very strigent noise/vibration criteria for the activities to not cause any disturbance to neighbours needing to rest or sleep

External noise levels are also lower between 19:00 and 07:00 hours, so the activities or the music are more audible to neighbours.

 

Note: This section might be redundant as a lot of sport, fitness and gym facilities operate 24/7 and their peak hours are generally early in the morning (between 06:00 and 08:00 hours) and in the evening (between 17:00 and 20:00 hours)

 

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What is the structure of the building where the facility is? 

 The structure of a building has a big influence in the acoustic treatment required.

Structure impact - Noise and vibration challenges in sport, fitness and gym facilities Part 1: Site Analysis - acoustic consultant - noise control - gym acoustics - vibration control - gym vibration

 

The structure of the building influences: 

 

  • The sound insulation treatment for the floors and the walls.
  • The vibration isolation treatment for the floors.

And you will generally need less material for heavy and stiff structures than for light structure. 

Examples of heavy structures are in-situ concrete structures, brickwork, etc.

Examples of light structures are timber, mass timber, pre-cast concrete and metal frame structures.

 

Note: If you want to install a free weight area on a timber structure, it will be very challenging, or expensive, to control the ‘structure-borne‘ noise generated by the impacts of the weights on the floor.

(see above Section Where are the noise sensitive receptors?  above for the definition of ‘structure-borne‘ noise) 

 

 

 

 

What is the layout of the facility? 

 (i.e. where will the different activities go)

Planning the facilities layout is an integral part of noise and vibration control strategy.

It may be obvious, but:

the further you locate the noisy and vibrating activities from the sensitive receptors,

the less materials and acoustic treatment you need.

So by just optimising the layout of a facility, you can already control the noise and vibration impacts of the activities.

Below are three types of areas that are worth considering for such as process.

Note: The last two points are particularly useful to know if you need to select different sites and assess their suitability for  sport, fitness and gym use.

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External running tracks, sport pitches and other  areas

Some external sport areas produce noise that can be particularly disturbing when they are close to sensitive receptors. 

Therefore, by locating these areas strategically, you can avoid some solutions like acoustic fences or earth bunds.

There are two main strategies here. Either you locate the areas:

  • away from the receptors, or;
  • ‘behind’ the some buildings (that of the facilities for example) that you use as acoustic screens.

Obviously, they are very much site dependant and sometimes can’t be implemented

 

studios - dance studio - sports hall indoor areas - sports halls - Noise and vibration challenges in sport, fitness and gym facilities Part 1: Site Analysis - acoustic consultant - noise control - gym acoustics - vibration control - gym vibration

Studios, sports halls and other indoor areas

Most studios, sports hall and other indoor areas organise noisy activities or events with loud amplified sounds.

They can be particularly disturbing to receptors within the same building or in a neighbouring building

Although you can control noise (and vibrations!) from these areas with the finishes and the construction of the building. It is also worth thinking about locating them as far away as possible from the receptors … when possible. 

Another solution is to use secondary spaces (like bathrooms, stores, locker rooms, etc) as ‘buffer’ areas between the noisy spaces and the receptors. 

free weight areas - barbell - kettlebell - treadmill - elliptical - gym bike - Vibrating machines - Noise and vibration challenges in sport, fitness and gym facilities Part 1: Site Analysis - acoustic consultant - noise control - gym acoustics - vibration control - gym vibration

‘Vibrant’ equipment and activities

 

The vibrations generated by some equipment and activities (such as  free weight activities, skate parks and even sports courts and studios) can be particuarly complicated and/or expensive to control.

Therefore, it will always be useful to think where they can go in the facilities to minimise the amounts of anti-vibration materials

  • at ground floor level and/or on heavy slabs.
  • As far away as possible from the sensitive receptors.

 

 

Suspended acoustic rafts explained

 

Have you ever wondered what suspended acoustic rafts are and how they acoustically work?

This page briefly explains:

  • what suspended acoustic rafts are exactly
  • which sound absorption performances acoustic rafts achieve and, most importantly, how they should be specified
  • which materials are usually used for acoustic rafts
  • examples of acoustics rafts products available on the market
 

 

Are you looking to understand how acoustic products work and find examples available on the market? 

Visit the Acoustic Design Catalogue by clicking on the button below.

 

What are suspended acoustic rafts?

Suspended acoustic rafts are free-hanging and flat elements made with a thick fibrous or porous materials that have sound absorption qualities. You hang them horizontally at a certain distance from a hard surface (usually a soffit).

They are a popular option to reduce the sound reverberation in:

  • education spaces
  • office spaces
  • assembly spaces
  • public spaces (such as receptions, entrance halls, etc)

 

Brief description of suspended acoustic raft systems

Suspended acoustic rafts - sound absorption - acoustic absorption - acoustic consultant - acoustic design - architectural acoustic design

Although acoustic rafts are mostly square or rectangular, some of them are also roundoval or any other bespoke shape.

Some raft manufacturers/suppliers give the option to include lighting and other electrical systems within the rafts.

 

Sound absorption performance of suspended acoustic rafts

The sound absorptive materials used for suspended acoustic rafts generally achieve sound absorption Class AClass B or Class C.

However, you should know that sound absorption classes are generally for a materials fixed to a hard surface with only one side visible. This side is the only one that absorbs sound. The other one doesn’t.

For acoustic rafts, it is a little bit different.

 

Suspended acoustic rafts – Sound absorption below and above

Not only the underside of the rafts absorbs sound, but also upper side indirectly.

Sound will first hit the hard surface above the acoustic rafts and then the upper side of the rafts.

Therefore, the distance between the rafts and the hard surface above has an influence on the sound absorption of the raft systems. Below approximately 1m, the further the rafts from the hard surface, the higher the sound absorption at mid and high frequencies.

 

Sound absorption below and above the rafts

sound absorption below and above the rafts - Suspended acoustic rafts - sound absorption - acoustic absorption - acoustic consultant - acoustic design - architectural acoustic design

Suspended acoustic rafts – Sound absorption of the edges

The edges of the rafts also absorb sound.

So a couple of large acoustic rafts absorb a little bit less sound than a few smaller acoustic rafts (with the same amount of absorptive materials).

 

Sound absorption of raft edges

sound absorptive edges - Suspended acoustic rafts - sound absorption - acoustic absorption - acoustic consultant - acoustic design - architectural acoustic design

Suspended acoustic rafts – Examples of sound absorption performances

To illustrate this, the graphs below shows examples of sound absorption performance for various raft systems.

 

Note: for rafts, the sound absorption performance is qualified in terms of equivalent absorption area per unit. Not in terms of sound absorption coefficient per m² of material.

 

Equivalent absorption area (m²) of 1200mm x 1200mm acoustic rafts located at different distances from the hard surface above 200 mm, 400 mm and 1000 mm and spaced 500 mm apart (Courtesy of Ecophon)

Equivalent absorption area (m²) of 1200mm x 1200mm acoustic rafts located at different distances from the hard surface above 200 mm, 400 mm and 1000 mm and spaced 500 mm apart (Courtesy of Ecophon) - sound absorptive edges - Suspended acoustic rafts - sound absorption - acoustic absorption - acoustic consultant - acoustic design - architectural acoustic design

 
 
Sound absorption per m² of raft based on rafts of different sizes hung at 1000mm from the hard surface above
(Courtesy of Ecophon)

Sound absorption per m² of raft based on rafts of different sizes hung at 1000mm from the hard surface above (Courtesy of Ecophon) - sound absorptive edges - Suspended acoustic rafts - sound absorption - acoustic absorption - acoustic consultant - acoustic design - architectural acoustic design

Suspended acoustic rafts – Specification

Based on the above, you can now understand that the sound absorption characteristics of raft systems don’t just depend on the material of the rafts.

Therefore, in the acoustic specifications, it is important to include the following information:

  • the sound absorption of the material installed in a room (when measured in line with ISO 354:2003 Acoustics — Measurement of sound absorption in a reverberation room)
  • the size of the rafts
  • the spacing between the rafts
  • the distance between the rafts and the hard surface above.

 

Note: the above is applicable to any other free hanging suspended acoustic system.

 

Materials used for suspended acoustic rafts

The materials preferred for acoustic rafts are usually thick fibrous or porous sound absorptive materials such as:

  • fibre glass
  • mineral wool
  • wood wool and mineral wool on top
  • polyester fibres
  • wood fibres

Sometimes, the sound absorptive material is wrapped in a fibrous fabric or even painted.

 

Examples of suspended acoustic rafts

To find examples of suspended acoustic raft products, visit the Acoustic Design Catalogue here.